Tapered electromagnetic horn



May 7, 1957 Filed April 8, 1946 FIG. I.

INVENTOR JACOB R. RISSER ATTORNEY United States Patent 6 TAPERED ELECTROMAGNETIC HORN Jacob R. Risser, Belmont, Mass.

Application April 8, 1946, Serial No. 660,581

6 Claims. (Cl. 343-786) This invention relates to an antenna, and more particularly to an electromagnetic horn having integral impedance matching means.

Electromagnetic horns are frequently used'as antennas or to excite a parabolic reflector type of antenna. Radiation pattern requirements usually dictate the dimensions of the aperture of the horn in the two major planes, i. e., the E-plane, or plane parallel to the vector direction of the electric component of the electromagnetic field produced by the horn, and the H-plane, or plane parallel to the vector direction of the magnetic component of the field.

Conventional compound horns, or' horns flared in both the E-plane and H-plane, are constructed with the flares starting at the same point along thev longitudinal axis of the horn, usually at th end of a waveguide transmission line.

It can be shown that since the conducting walls of the horn determine the boundary conditions for the electromagnetic field existing within the lrorn, the presence of a discontinuity in those walls perpendicular to the vector direction of the electric component of the field will cause a reflection of energy back into the transmission line. Thus, it can be seen that the discontinuity produced by the sudden change in distance between the walls in the E-plane caused by the beginning of the flare and the open .mouth of the horn, will result in reflections of energy and consequent loss of transferred power.

It is therefore the primary object of this invention to provide an electromagnetic horn including means for reducing the effects of impedance discontinuities in the horn.

It is another object of this invention to provide means for reducing the effects of a discontinuity at the edge of a conducting surface when such a surface determines certain boundary conditions for an electromagnetic field.

These and other objects will be more apparent upon consideration of the following description, together with the accompanying drawings in which:

Fig. l is a perspective view of an embodiment of the invention; and

Fig. 2 is a cross section along line IIII of Fig. 1 and illustrates the principles involved in the invention.

As shown in Fig. 1, the wide walls of a rectangular wave guide transmission line 10 are extended by parallel tapered walls 13 and 14 to form two sides of an electromagnetic horn flared in the H-plane. Tapered flat plates 15 and 16 are disposed at an angle to walls 13 and 14, forming extensions thereto and, at the same time, a flare in the E-plane. Strips 11 and 12 are fastened to wave guide 10, walls 13 and 14, and plates 15 and 16, to form the narrow E-plane walls of the electromagnetic horn.

The dimensions of the aperture in each plane are determined by the radiation pattern requirements, following the well-known design techniques. Although the horn shown in Fig. 1 is flared in the H-plane, it is not intended that the invention shall be so restricted. It can be shown that the impedance presented by an electromagnetic horn to a transmission line is relativelyindependent 'of the degree of flare in the H-plane, butis determined primarily by the parameters of the E-pla'ne flare.

The principles involved in the invention can be seen easily by referring to Fig. 2, which' isan E'-pl'a'ne"cross section along line II'-H of the embodiment shown in Fig. 1. It is to be assumed that the wave guide 10 is properly energized so that the fundamental mode of open ation exists, i. e., the vector direction of the electric component of the electromagnetic field is parallel to'the narrow walls of wave guide 10.

For the purpose of explanation, the horn shown in cross-section in Fig. 2 will be considered symmetrical, with the effects of both'sides' being identical. As shown, plate 15' is displaced at an angle 0' from the longitudinal axis of the horn. An electromagnetic wave A, represented by a dashed line and being propagated toward the mouth of the horn, will encounter an impedance discontinuity at the junction between 'wall' 13 and plate 15. The presence of this discontinuity will causethereflection back toward wave guide 10 of adeterminable fraction B of theincident energy contained in wave A. The incident wave A, minus the, reflected wave B, will continueto travel toward the mouth of the horn, where it will encounter a second discontinuity, i. e., that occurring atthe open mouth dueto the impedance differential existing between the horn and free space. A second reflected wave C of determinable amplitude'is returned back into the horn from this second discontinuity.

The width L of plate 1 5 is made such that the electrical distance between the effective points of reflection is an odd multiple of a quarter wave length ofjthe' electromagnetic energy propagated within the horn; The relative time phases of the reflected waves B and C are then such that the combination results in a reduction of the amplitude of the wave reflected back into wave guide 10. The reflected waves B and C can be made to have substantially equal amplitudes by proper choice of the flare angle 0. Thus it can be seen that a selected combination of angle 0 and width L will produce cancellation and effectively terminate wave guide 10 properly.

It has been determined that the best impedance match is effected when angle 0 is less than 40 and width L is approximately two-tenths, rather than precisely a quarter of a wave length of the energy propagated in the E-plane flare or two-tenths of a wave length plus any integral number of one-half wave lengths. The wave length used to determine the width L depends upon the distance between the walls 11 and 12 in the H-plane and is determined in the well-known manner.

While the above discussion was limited to the application of the invention to electromagnetic horns, it is not intended that the invention shall be so limited. The principles described can be applied to any conductive surface over which electromagnetic energy is caused to flow, such as, for example, metallic surfaces positioned in close proximity to a radiated electromagnetic field, where the presence of such surfaces produces undesirable distortion of the field configuration.

Since certain changes may be made in the above described apparatus, and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying draw ings shall be interpreted as illustrative and not in a limiting sense, and therefore that the invention is to be limited only by the spirit of the appended claims.

What is claimed is:

1. An electromagnetic horn comprising, a wave guide transmission line having a rectangular cross section, first and second parallel wedge-shaped conductive walls dis- M. EAT

it ,,.J-- V 3 posed in the planes of the wide walls of said wave guide and forming extensions thereof, first and second tapered fiat plates disposed at predetermined angles to said first and second walls and forming a flared section, first .and second conductive strips fastened to the narrow walls of said wave guide, saidstrips being fastened to the edges of said first and second'walls and said plates, said strips forming the narrow walls of said horn, said predetermined angles being-eflective to produce at the junction between said first and second walls and said flat plates and at the mouth of said horn reflections of electromagnetic energy having substantially equal magnitudes, the distance between said junction and said mouthbeing substantially equal to two-tenths of a wave length of said energy.

2. An electromagnetic horn comprising, a flared section of rectangular wave guide, the origin of the flare in one pair of opposite wallsof said wave guide being longitudinally displaced along said wave guide from the origin of the flare in the other pair of opposite Walls, the length ,of the flare of one of said pairs of opposite walls being equal to two-tenths of a wave length of energy at the frequency of operation of said wave guide and the angle of the flare of each of the opposing walls of one of said pairs being less than fortydegrees relative to the longitudinal axis of said wave guide. 7

3. An electromagnetic horn comprising, a flared section ,ofrectangular wave guide, the flare in one pair of opposite walls. of said Wave guide originating at a difierent location along said wave guide than the flare in the other pair of opposite walls, the length of the flare of one of said pairs of opposite walls being equal to twotenths of a wave length of energy at the frequency of operation of said wave guide, the angle of the flare of .each of the opposing walls of the pair flared nearer the large end of said flared section being less than forty degrees relative to the longitudinal axis of said rectangular Wave guide.

4. An electromagnetic horn of generally pyramidal configuration having throat and mouth orifices, said horn being symmetrically tapered in the H-plane from the mouth to the throat orifice and being symmetrically tapered in the E-plane from the mouth to an intermediate point of said born, the slant length of said E-plane taper being approximately equal to two-tenths of a wave length of the electromagnetic energy being propagated within said horn whereby electromagnetic energy reflected back towards the throat from the mouth and from said intermediate point are in an out-of-phase relationship.

5. In an electromagnetic horn as defined in claim 4 wherein said Erplane taper is at an angle less than forty degrees with the longitudinal axis of said born.

6. An electromagnetic horn of generally pyramidal configuration comprising, a section of rectangular wave guide,

References Cited in the file of this patent UNITED STATES PATENTS 2,316,151 Barrow Apr. 13, 1943 2,479,673 Devore Aug. 23, 1949 2,549,721 Straus Apr. 17, 1951 2,611,937 Van Atta Nov. 11, 1952 FOREIGN PATENTS 238,928 Switzerland Nov. 16, 1945 

